Signaling missing sections of media data for network streaming in a manifest file

ABSTRACT

An example device for retrieving media data includes a memory configured to store media data; and one or more processors implemented in circuitry and configured to: determine that a manifest file for a presentation of the media data includes data specifying a period of time of the presentation for which media data is not available but for which a segment timeline can be extended; retrieve a first set of segments of the presentation including media data having presentation times prior to the period of time; retrieve a second set of segments of the presentation including media data having presentation times after the period of time; in response to the determination that the manifest file includes the data specifying the period of time: omit retrieval of media data having presentation times during the period of time; and extend the segment timeline according to the determination.

This application claims the benefit of U.S. Provisional Application No.62/670,370, filed May 11, 2018, the entire contents of which are herebyincorporated by reference.

TECHNICAL FIELD

This disclosure relates to transport of encoded media data.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, digital cameras, digital recording devices,digital media players, video gaming devices, video game consoles,cellular or satellite radio telephones, video teleconferencing devices,and the like. Digital video devices implement video compressiontechniques, such as those described in the standards defined by MPEG-2,MPEG-4, ITU-T H.263 or ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265 (also referred to as High Efficiency VideoCoding (HEVC)), and extensions of such standards, to transmit andreceive digital video information more efficiently.

After video data has been encoded, the video data may be packetized fortransmission or storage. The video data may be assembled into a videofile conforming to any of a variety of standards, such as theInternational Organization for Standardization (ISO) base media fileformat and extensions thereof, such as AVC.

SUMMARY

In general, this disclosure describes techniques for signaling missingsections of media data for network streaming services, such as DynamicAdaptive Streaming over HTTP (DASH). In some cases, expected media dataof segments may not be present due to failures in different parts of astreaming system. In accordance with the techniques of this disclosure,such expected media data may be signaled in a segment as missingsections, which allow extension of a segment timeline in a receivingdevice. That is, the receiving device can retrieve media data havingpresentation times after the missing section and output the media data,without outputting desired media data for the missing section.Additionally or alternatively, missing sections may be signaled in amanifest file, such that receiving devices may avoid requesting themissing sections.

In one example, a method of retrieving media data includes determiningthat a manifest file for a presentation of media data includes dataspecifying a period of time of the presentation for which media data isnot available but for which a segment timeline can be extended;retrieving a first set of segments of the presentation including mediadata having presentation times prior to the period of time; in responseto the determination that the manifest file includes the data specifyingthe period of time: omitting retrieval of media data having presentationtimes during the period of time; retrieving a second set of segments ofthe presentation including media data having presentation times afterthe period of time; and extending the segment timeline according to thedetermination.

In another example, a device for retrieving media data includes a memoryconfigured to store media data; and one or more processors implementedin circuitry and configured to: determine that a manifest file for apresentation of the media data includes data specifying a period of timeof the presentation for which media data is not available but for whicha segment timeline can be extended; retrieve a first set of segments ofthe presentation including media data having presentation times prior tothe period of time; retrieve a second set of segments of thepresentation including media data having presentation times after theperiod of time; in response to the determination that the manifest fileincludes the data specifying the period of time: omit retrieval of mediadata having presentation times during the period of time; and extend thesegment timeline according to the determination.

In another example, a computer-readable storage medium has storedthereon instructions that, when executed, cause a processor to determinethat a manifest file for a presentation of media data includes dataspecifying a period of time of the presentation for which media data isnot available but for which a segment timeline can be extended; retrievea first set of segments of the presentation including media data havingpresentation times prior to the period of time; retrieve a second set ofsegments of the presentation including media data having presentationtimes after the period of time; in response to the determination thatthe manifest file includes the data specifying the period of time: omitretrieval of media data having presentation times during the period oftime; and extend the segment timeline according to the determination.

In another example, a device for retrieving media data includes meansfor determining that a manifest file for a presentation of media dataincludes data specifying a period of time of the presentation for whichmedia data is not available but for which a segment timeline can beextended; means for retrieving a first set of segments of thepresentation including media data having presentation times prior to theperiod of time; means for retrieving a second set of segments of thepresentation including media data having presentation times after theperiod of time; means for omitting retrieval of media data havingpresentation times during the period of time in response to thedetermination that the manifest file includes the data specifying theperiod of time; and means for extending the segment timeline accordingto the determination in response to the determination that the manifestfile includes the data specifying the period of time.

In another example, a method of retrieving media data includesretrieving at least a portion of a first segment of a representation ofmedia data, the at least portion of the first segment including dataindicating that the first segment includes a missing sectionrepresenting a period of time for which media data is not available butfor which a segment timeline can be extended; retrieving media data of asecond segment having a presentation time following the missing section;extending the segment timeline according to the data of the at leastportion of the first segment; and outputting the media data of thesecond segment according to the extended segment timeline.

In another example, a device for retrieving media data includes a memoryconfigured to store media data; and one or more processors implementedin circuitry and configured to: retrieve at least a portion of a firstsegment of a representation of the media data, the at least portion ofthe first segment including data indicating that the first segmentincludes a missing section representing a period of time for which mediadata is not available but for which a segment timeline can be extended;retrieve media data of a second segment having a presentation timefollowing the missing section; extend the segment timeline according tothe data of the at least portion of the first segment; and output themedia data of the second segment according to the extended segmenttimeline.

In another example, a computer-readable storage medium has storedthereon instructions that, when executed, cause a processor to retrieveat least a portion of a first segment of a representation of the mediadata, the at least portion of the first segment including dataindicating that the first segment includes a missing sectionrepresenting a period of time for which media data is not available butfor which a segment timeline can be extended; retrieve media data of asecond segment having a presentation time following the missing section;extend the segment timeline according to the data of the at leastportion of the first segment; and output the media data of the secondsegment according to the extended segment timeline.

In another example, a device for retrieving media data includes meansfor retrieving at least a portion of a first segment of a representationof media data, the at least portion of the first segment including dataindicating that the first segment includes a missing sectionrepresenting a period of time for which media data is not available butfor which a segment timeline can be extended; means for retrieving mediadata of a second segment having a presentation time following themissing section; means for extending the segment timeline according tothe data of the at least portion of the first segment; and means foroutputting the media data of the second segment according to theextended segment timeline.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example system that implementstechniques for streaming media data over a network.

FIG. 2 is a block diagram illustrating an example set of components of aretrieval unit in greater detail.

FIG. 3 is a conceptual diagram illustrating elements of examplemultimedia content.

FIG. 4 is a block diagram illustrating elements of an example videofile.

FIG. 5 is a conceptual diagram illustrating various examples ofscenarios in which loss of media data of one or more segments may occur.

FIG. 6 is a flowchart illustrating an example method of determining thatstreamed media data includes a missing section, according to thetechniques of this disclosure.

FIG. 7 is a flowchart illustrating an example of a method for retrievinga manifest file including data indicating that a presentation includes amissing section, according to the techniques of this disclosure.

FIG. 8 is a flowchart illustrating an example of a method for retrievinga segment including data indicating that the segment includes a missingsection, according to the techniques of this disclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for signaling missingsections of media data for network streaming services, such as DynamicAdaptive Streaming over HTTP (DASH), HTTP Live Streaming (HLS),Microsoft Smooth Streaming, or the like.

In case of linear (live) distribution, expected segments may notnecessarily be present due to failures in different parts of theend-to-end system. A good overview of use cases appears in Section 4.8of DASH Industry Forum, “Guidelines for Implementation: DASH-IFInteroperability Points,” Version 4, Dec. 12, 2016, available atdashif.org/wp-content/uploads/2016/12/DASH-IF-IOP-v4.0-clean.pdf.

One such example is encoder/packager failures. If an encoder/packagerfails for some reason, and a redundant encoder/packager startsoutputting segments, there may be a gap of one or more segments betweenthe last segment written to origin by the primary encoder/packager andthe first segment written by the redundant encoder/packager.

Another example is when the encoder output is an MPEG-2 TS User DatagramProtocol (UDP) multicast with virtual segmentation such as CableLabs EBPor MPEG-2 boundary descriptors. This approach does not use reliablemulticast protocols. A UDP packet from such deployment may contain sevenMPEG-2 TS packets and no other data. When losses occur, segments may notbe produced by the downstream packagers.

Yet another example is when different representations are generated bydifferent encoders/packagers. In this case, when an encoder fails, theremay be a gap only for one or more of the representations, but theremaining representation(s) will still have an available segment.

Two example issues may occur as a result of the discussion above. In oneexample, if no segment is produced, then a request (e.g., an HTTP GET orpartial GET request) for such a segment may result in an error, such asan HTTP 404 error. Producing a broad set of 404 errors in operation isundesirable, as it generates unnecessary operational burden for contentdelivery networks (CDNs). Secondly, without the production of a segment,at least in certain cases, the operational flow of the client may beinterrupted, especially when the DASH main live client according to theDASH-IF TOP is used. The loss of a segment results that no longer eventmessage (EMSG) and no longer the segment information can be distributedas document in DASH-IF TOP v. 4.0, clause 4.5.3, in order to continueoperation. The DASH client may then rely on an MPD update to continueunderstanding the details (e.g., subsequent media data).

To address these issues, DASH provides a mechanism to cope with segmentlosses using the SegmentTimeline element. A gap can be calculated fromthe difference in the expected value of S@t calculated from the previousS elements, and the value of S@t. However, a potential problem of thisapproach is that, in the case of segment losses in a representation, aSegmentTimeline at the representation level may be used, rather than atthe adaptation set level as it is typically done. Using SegmentTimelineat the adaptation set level would apply to all the representations,which would be incorrect in case the loss is specific to a subset of therepresentations.

Use of representation-level SegmentTimeline adds extra XML elements toevery representation. This works well in case of segments of preciselythe same duration as large @r values can reduce the number of S elementsneeded to express all of the segments. When segments of similar but notequal durations are used (e.g., 2-second segments with +/−1 secondvariation), SegmentTimeline may quickly grow fairly large. Given theneed to repeat the same information for all representations within anadaptation set, and in case of a large number of representations (e.g.,14), this results in very large manifest files such as mediapresentation descriptions (MPDs), both in bytes and in number of XMLelements. The negative effect of large MPD sizes is network overhead.The negative effects of a large number of XML elements is the increasein the MPD parse time and memory usage on the client side.

The techniques of this disclosure generally concern using a commonSegmentTimeline at the adaptation set level (which applies to all therepresentations in this adaptation set). This allows for a relativelysmaller MPD in size with fewer elements compared to signalingrepresentation-level SegmentTimeline elements, but having arepresentation-level indication for missing sections (e.g., withinsegments or full missing segments).

Additionally or alternatively, a content preparation device may add adummy segment to replace missing information. For example, an encoder,encapsulation unit, DASH packager, or the like may produce a wellidentified segment that is marked such that a DASH client can identifythe segment as missing or non-complete. Therefore, the DASH client maysend data to a corresponding media decoder indicating the issue, i.e.,that media data is missing. As another option, the DASH packager mayproduce a valid segment for which the content may represent a blackscreen or one or more error messages. In both cases, this addition maycause confusion by the viewer. Thus, a client device may avoiddisplaying the error messages or black screen if other representationsprovide valid alternatives. Therefore, even such segments and mediasections may be avoided. For this purpose, the DASH client may need tobe aware of the dummy segment or well identified error-indicativesegment.

In general, the techniques of this disclosure include two aspects.First, in the live case, a content generation device (e.g., anencapsulation unit thereof) may generate and insert segments that arenot complete or entirely missing and that are clearly marked ascontaining a missing section (e.g., the segment is a missing sectionsegment), but permit extension of the timeline in a receiver devicewithout manifest file updates (such as MPD updates). Second, signalingfor missing sections may be included in the manifest file (such as anMPD) such that clients (e.g., receiving devices) that have access to theMPD can make use of the information and avoid requesting the segmentswith missing information. Segments with “missing” information may referto segments as discussed above (e.g., segments that are not complete orare entirely missing and marked as such), or may refer to content thatis not desired main media content, such as a black screen or errormessages.

The techniques of this disclosure may be applied to video filesconforming to video data encapsulated according to any of ISO base mediafile format, Scalable Video Coding (SVC) file format, Advanced VideoCoding (AVC) file format, Third Generation Partnership Project (3GPP)file format, and/or Multiview Video Coding (MVC) file format, or othersimilar video file formats.

In HTTP streaming, frequently used operations include HEAD, GET, andpartial GET. The HEAD operation retrieves a header of a file associatedwith a given uniform resource locator (URL) or uniform resource name(URN), without retrieving a payload associated with the URL or URN. TheGET operation retrieves a whole file associated with a given URL or URN.The partial GET operation receives a byte range as an input parameterand retrieves a continuous number of bytes of a file, where the numberof bytes correspond to the received byte range. Thus, movie fragmentsmay be provided for HTTP streaming, because a partial GET operation canget one or more individual movie fragments. In a movie fragment, therecan be several track fragments of different tracks. In HTTP streaming, amedia presentation may be a structured collection of data that isaccessible to the client. The client may request and download media datainformation to present a streaming service to a user.

In the example of streaming 3GPP data using HTTP streaming, there may bemultiple representations for video and/or audio data of multimediacontent. As explained below, different representations may correspond todifferent coding characteristics (e.g., different profiles or levels ofa video coding standard), different coding standards or extensions ofcoding standards (such as multiview and/or scalable extensions), ordifferent bitrates. The manifest of such representations may be definedin a Media Presentation Description (MPD) data structure. A mediapresentation may correspond to a structured collection of data that isaccessible to an HTTP streaming client device. The HTTP streaming clientdevice may request and download media data information to present astreaming service to a user of the client device. A media presentationmay be described in the MPD data structure, which may include updates ofthe MPD.

A media presentation may contain a sequence of one or more Periods. Eachperiod may extend until the start of the next Period, or until the endof the media presentation, in the case of the last period. Each periodmay contain one or more representations for the same media content. Arepresentation may be one of a number of alternative encoded versions ofaudio, video, timed text, or other such data. The representations maydiffer by encoding types, e.g., by bitrate, resolution, and/or codec forvideo data and bitrate, language, and/or codec for audio data. The termrepresentation may be used to refer to a section of encoded audio orvideo data corresponding to a particular period of the multimediacontent and encoded in a particular way.

Representations of a particular period may be assigned to a groupindicated by an attribute in the MPD indicative of an adaptation set towhich the representations belong. Representations in the same adaptationset are generally considered alternatives to each other, in that aclient device can dynamically and seamlessly switch between theserepresentations, e.g., to perform bandwidth adaptation. For example,each representation of video data for a particular period may beassigned to the same adaptation set, such that any of therepresentations may be selected for decoding to present media data, suchas video data or audio data, of the multimedia content for thecorresponding period. The media content within one period may berepresented by either one representation from group 0, if present, orthe combination of at most one representation from each non-zero group,in some examples. Timing data for each representation of a period may beexpressed relative to the start time of the period.

A representation may include one or more segments. Each representationmay include an initialization segment, or each segment of arepresentation may be self-initializing. When present, theinitialization segment may contain initialization information foraccessing the representation. In general, the initialization segmentdoes not contain media data. A segment may be uniquely referenced by anidentifier, such as a uniform resource locator (URL), uniform resourcename (URN), or uniform resource identifier (URI). The MPD may providethe identifiers for each segment. In some examples, the MPD may alsoprovide byte ranges in the form of a range attribute, which maycorrespond to the data for a segment within a file accessible by theURL, URN, or URI.

Different representations may be selected for substantially simultaneousretrieval for different types of media data. For example, a clientdevice may select an audio representation, a video representation, and atimed text representation from which to retrieve segments. In someexamples, the client device may select particular adaptation sets forperforming bandwidth adaptation. That is, the client device may selectan adaptation set including video representations, an adaptation setincluding audio representations, and/or an adaptation set includingtimed text. Alternatively, the client device may select adaptation setsfor certain types of media (e.g., video), and directly selectrepresentations for other types of media (e.g., audio and/or timedtext).

FIG. 1 is a block diagram illustrating an example system 10 thatimplements techniques for streaming media data over a network. In thisexample, system 10 includes content preparation device 20, server device60, and client device 40. Client device 40 and server device 60 arecommunicatively coupled by network 74, which may comprise the Internet.In some examples, content preparation device 20 and server device 60 mayalso be coupled by network 74 or another network, or may be directlycommunicatively coupled. In some examples, content preparation device 20and server device 60 may comprise the same device.

Content preparation device 20, in the example of FIG. 1, comprises audiosource 22 and video source 24. Audio source 22 may comprise, forexample, a microphone that produces electrical signals representative ofcaptured audio data to be encoded by audio encoder 26. Alternatively,audio source 22 may comprise a storage medium storing previouslyrecorded audio data, an audio data generator such as a computerizedsynthesizer, or any other source of audio data. Video source 24 maycomprise a video camera that produces video data to be encoded by videoencoder 28, a storage medium encoded with previously recorded videodata, a video data generation unit such as a computer graphics source,or any other source of video data. Content preparation device 20 is notnecessarily communicatively coupled to server device 60 in all examples,but may store multimedia content to a separate medium that is read byserver device 60.

Raw audio and video data may comprise analog or digital data. Analogdata may be digitized before being encoded by audio encoder 26 and/orvideo encoder 28. Audio source 22 may obtain audio data from a speakingparticipant while the speaking participant is speaking, and video source24 may simultaneously obtain video data of the speaking participant. Inother examples, audio source 22 may comprise a computer-readable storagemedium comprising stored audio data, and video source 24 may comprise acomputer-readable storage medium comprising stored video data. In thismanner, the techniques described in this disclosure may be applied tolive, streaming, real-time audio and video data or to archived,pre-recorded audio and video data.

Audio frames that correspond to video frames are generally audio framescontaining audio data that was captured (or generated) by audio source22 contemporaneously with video data captured (or generated) by videosource 24 that is contained within the video frames. For example, whilea speaking participant generally produces audio data by speaking, audiosource 22 captures the audio data, and video source 24 captures videodata of the speaking participant at the same time, that is, while audiosource 22 is capturing the audio data. Hence, an audio frame maytemporally correspond to one or more particular video frames.Accordingly, an audio frame corresponding to a video frame generallycorresponds to a situation in which audio data and video data werecaptured at the same time and for which an audio frame and a video framecomprise, respectively, the audio data and the video data that wascaptured at the same time.

In some examples, audio encoder 26 may encode a timestamp in eachencoded audio frame that represents a time at which the audio data forthe encoded audio frame was recorded, and similarly, video encoder 28may encode a timestamp in each encoded video frame that represents atime at which the video data for an encoded video frame was recorded. Insuch examples, an audio frame corresponding to a video frame maycomprise an audio frame comprising a timestamp and a video framecomprising the same timestamp. Content preparation device 20 may includean internal clock from which audio encoder 26 and/or video encoder 28may generate the timestamps, or that audio source 22 and video source 24may use to associate audio and video data, respectively, with atimestamp.

In some examples, audio source 22 may send data to audio encoder 26corresponding to a time at which audio data was recorded, and videosource 24 may send data to video encoder 28 corresponding to a time atwhich video data was recorded. In some examples, audio encoder 26 mayencode a sequence identifier in encoded audio data to indicate arelative temporal ordering of encoded audio data but without necessarilyindicating an absolute time at which the audio data was recorded, andsimilarly, video encoder 28 may also use sequence identifiers toindicate a relative temporal ordering of encoded video data. Similarly,in some examples, a sequence identifier may be mapped or otherwisecorrelated with a timestamp.

Audio encoder 26 generally produces a stream of encoded audio data,while video encoder 28 produces a stream of encoded video data. Eachindividual stream of data (whether audio or video) may be referred to asan elementary stream. An elementary stream is a single, digitally coded(possibly compressed) component of a representation. For example, thecoded video or audio part of the representation can be an elementarystream. An elementary stream may be converted into a packetizedelementary stream (PES) before being encapsulated within a video file.Within the same representation, a stream ID may be used to distinguishthe PES-packets belonging to one elementary stream from the other. Thebasic unit of data of an elementary stream is a packetized elementarystream (PES) packet. Thus, coded video data generally corresponds toelementary video streams. Similarly, audio data corresponds to one ormore respective elementary streams.

Many video coding standards, such as ITU-T H.264/AVC and the upcomingHigh Efficiency Video Coding (HEVC) standard, define the syntax,semantics, and decoding process for error-free bitstreams, any of whichconform to a certain profile or level. Video coding standards typicallydo not specify the encoder, but the encoder is tasked with guaranteeingthat the generated bitstreams are standard-compliant for a decoder. Inthe context of video coding standards, a “profile” corresponds to asubset of algorithms, features, or tools and constraints that apply tothem. As defined by the H.264 standard, for example, a “profile” is asubset of the entire bitstream syntax that is specified by the H.264standard. A “level” corresponds to the limitations of the decoderresource consumption, such as, for example, decoder memory andcomputation, which are related to the resolution of the pictures, bitrate, and block processing rate. A profile may be signaled with aprofile_idc (profile indicator) value, while a level may be signaledwith a level_idc (level indicator) value.

The H.264 standard, for example, recognizes that, within the boundsimposed by the syntax of a given profile, it is still possible torequire a large variation in the performance of encoders and decodersdepending upon the values taken by syntax elements in the bitstream suchas the specified size of the decoded pictures. The H.264 standardfurther recognizes that, in many applications, it is neither practicalnor economical to implement a decoder capable of dealing with allhypothetical uses of the syntax within a particular profile.Accordingly, the H.264 standard defines a “level” as a specified set ofconstraints imposed on values of the syntax elements in the bitstream.These constraints may be simple limits on values. Alternatively, theseconstraints may take the form of constraints on arithmetic combinationsof values (e.g., picture width multiplied by picture height multipliedby number of pictures decoded per second). The H.264 standard furtherprovides that individual implementations may support a different levelfor each supported profile.

A decoder conforming to a profile ordinarily supports all the featuresdefined in the profile. For example, as a coding feature, B-picturecoding is not supported in the baseline profile of H.264/AVC but issupported in other profiles of H.264/AVC. A decoder conforming to alevel should be capable of decoding any bitstream that does not requireresources beyond the limitations defined in the level. Definitions ofprofiles and levels may be helpful for interpretability. For example,during video transmission, a pair of profile and level definitions maybe negotiated and agreed for a whole transmission session. Morespecifically, in H.264/AVC, a level may define limitations on the numberof macroblocks that need to be processed, decoded picture buffer (DPB)size, coded picture buffer (CPB) size, vertical motion vector range,maximum number of motion vectors per two consecutive MBs, and whether aB-block can have sub-macroblock partitions less than 8×8 pixels. In thismanner, a decoder may determine whether the decoder is capable ofproperly decoding the bitstream.

In the example of FIG. 1, encapsulation unit 30 of content preparationdevice 20 receives elementary streams comprising coded video data fromvideo encoder 28 and elementary streams comprising coded audio data fromaudio encoder 26. In some examples, video encoder 28 and audio encoder26 may each include packetizers for forming PES packets from encodeddata. In other examples, video encoder 28 and audio encoder 26 may eachinterface with respective packetizers for forming PES packets fromencoded data. In still other examples, encapsulation unit 30 may includepacketizers for forming PES packets from encoded audio and video data.

Video encoder 28 may encode video data of multimedia content in avariety of ways, to produce different representations of the multimediacontent at various bitrates and with various characteristics, such aspixel resolutions, frame rates, conformance to various coding standards,conformance to various profiles and/or levels of profiles for variouscoding standards, representations having one or multiple views (e.g.,for two-dimensional or three-dimensional playback), or other suchcharacteristics. A representation, as used in this disclosure, maycomprise one of audio data, video data, text data (e.g., for closedcaptions), or other such data. The representation may include anelementary stream, such as an audio elementary stream or a videoelementary stream. Each PES packet may include a stream_id thatidentifies the elementary stream to which the PES packet belongs.Encapsulation unit 30 is responsible for assembling elementary streamsinto video files (e.g., segments) of various representations.

Encapsulation unit 30 receives PES packets for elementary streams of arepresentation from audio encoder 26 and video encoder 28 and formscorresponding network abstraction layer (NAL) units from the PESpackets. Coded video segments may be organized into NAL units, whichprovide a “network-friendly” video representation addressingapplications such as video telephony, storage, broadcast, or streaming.NAL units can be categorized to Video Coding Layer (VCL) NAL units andnon-VCL NAL units. VCL units may contain the core compression engine andmay include block, macroblock, and/or slice level data. Other NAL unitsmay be non-VCL NAL units. In some examples, a coded picture in one timeinstance, normally presented as a primary coded picture, may becontained in an access unit, which may include one or more NAL units.

Non-VCL NAL units may include parameter set NAL units and SEI NAL units,among others. Parameter sets may contain sequence-level headerinformation (in sequence parameter sets (SPS)) and the infrequentlychanging picture-level header information (in picture parameter sets(PPS)). With parameter sets (e.g., PPS and SPS), infrequently changinginformation need not to be repeated for each sequence or picture; hence,coding efficiency may be improved. Furthermore, the use of parametersets may enable out-of-band transmission of the important headerinformation, avoiding the need for redundant transmissions for errorresilience. In out-of-band transmission examples, parameter set NALunits may be transmitted on a different channel than other NAL units,such as SEI NAL units.

Supplemental Enhancement Information (SEI) may contain information thatis not necessary for decoding the coded pictures samples from VCL NALunits, but may assist in processes related to decoding, display, errorresilience, and other purposes. SEI messages may be contained in non-VCLNAL units. SEI messages are the normative part of some standardspecifications, and thus are not always mandatory for standard compliantdecoder implementation. SEI messages may be sequence level SEI messagesor picture level SEI messages. Some sequence level information may becontained in SEI messages, such as scalability information SEI messagesin the example of SVC and view scalability information SEI messages inMVC. These example SEI messages may convey information on, e.g.,extraction of operation points and characteristics of the operationpoints. In addition, encapsulation unit 30 may form a manifest file,such as a media presentation descriptor (MPD) that describescharacteristics of the representations. Encapsulation unit 30 may formatthe MPD according to extensible markup language (XML).

Encapsulation unit 30 may provide data for one or more representationsof multimedia content, along with the manifest file (e.g., the MPD) tooutput interface 32. Output interface 32 may comprise a networkinterface or an interface for writing to a storage medium, such as auniversal serial bus (USB) interface, a CD or DVD writer or burner, aninterface to magnetic or flash storage media, or other interfaces forstoring or transmitting media data. Encapsulation unit 30 may providedata of each of the representations of multimedia content to outputinterface 32, which may send the data to server device 60 via networktransmission or storage media. In the example of FIG. 1, server device60 includes storage medium 62 that stores various multimedia contents64, each including a respective manifest file 66 and one or morerepresentations 68A-68N (representations 68). In some examples, outputinterface 32 may also send data directly to network 74.

In some cases, such as live streaming, various components of contentpreparation device 20 may encounter an error, as discussed above.Various error scenarios are discussed in Section 4.8 of DASH-IF TOP v.4, as noted above. Encapsulation unit 30 may detect such an error andperform various techniques of this disclosure in response. For example,encapsulation unit 30 may generate small segments that are marked asmissing section segments. Additionally or alternatively, encapsulationunit 30 may generate manifest file 66 to include information indicatingthe missing section segments, such that client device 40 may use theinformation of manifest file 66 to avoid requesting the missing sectionsegments, to avoid wasting bandwidth of, e.g., network 74.

In some examples, representations 68 may be separated into adaptationsets. That is, various subsets of representations 68 may includerespective common sets of characteristics, such as codec, profile andlevel, resolution, number of views, file format for segments, text typeinformation that may identify a language or other characteristics oftext to be displayed with the representation and/or audio data to bedecoded and presented, e.g., by speakers, camera angle information thatmay describe a camera angle or real-world camera perspective of a scenefor representations in the adaptation set, rating information thatdescribes content suitability for particular audiences, or the like.

Manifest file 66 may include data indicative of the subsets ofrepresentations 68 corresponding to particular adaptation sets, as wellas common characteristics for the adaptation sets. Manifest file 66 mayalso include data representative of individual characteristics, such asbitrates, for individual representations of adaptation sets. In thismanner, an adaptation set may provide for simplified network bandwidthadaptation. Representations in an adaptation set may be indicated usingchild elements of an adaptation set element of manifest file 66.

Server device 60 includes request processing unit 70 and networkinterface 72. In some examples, server device 60 may include a pluralityof network interfaces. Furthermore, any or all of the features of serverdevice 60 may be implemented on other devices of a content deliverynetwork, such as routers, bridges, proxy devices, switches, or otherdevices. In some examples, intermediate devices of a content deliverynetwork may cache data of multimedia content 64, and include componentsthat conform substantially to those of server device 60. In general,network interface 72 is configured to send and receive data via network74.

Request processing unit 70 is configured to receive network requestsfrom client devices, such as client device 40, for data of storagemedium 62. For example, request processing unit 70 may implementhypertext transfer protocol (HTTP) version 1.1, as described in RFC2616, “Hypertext Transfer Protocol—HTTP/1.1,” by R. Fielding et al,Network Working Group, IETF, June 1999. That is, request processing unit70 may be configured to receive HTTP GET or partial GET requests andprovide data of multimedia content 64 in response to the requests. Therequests may specify a segment of one of representations 68, e.g., usinga URL of the segment. In some examples, the requests may also specifyone or more byte ranges of the segment, thus comprising partial GETrequests. Request processing unit 70 may further be configured toservice HTTP HEAD requests to provide header data of a segment of one ofrepresentations 68. In any case, request processing unit 70 may beconfigured to process the requests to provide requested data to arequesting device, such as client device 40.

Additionally or alternatively, request processing unit 70 may beconfigured to deliver media data via a broadcast or multicast protocol,such as eMBMS. Content preparation device 20 may create DASH segmentsand/or sub-segments in substantially the same way as described, butserver device 60 may deliver these segments or sub-segments using eMBMSor another broadcast or multicast network transport protocol. Forexample, request processing unit 70 may be configured to receive amulticast group join request from client device 40. That is, serverdevice 60 may advertise an Internet protocol (IP) address associatedwith a multicast group to client devices, including client device 40,associated with particular media content (e.g., a broadcast of a liveevent). Client device 40, in turn, may submit a request to join themulticast group. This request may be propagated throughout network 74,e.g., routers making up network 74, such that the routers are caused todirect traffic destined for the IP address associated with the multicastgroup to subscribing client devices, such as client device 40.

As illustrated in the example of FIG. 1, multimedia content 64 includesmanifest file 66, which may correspond to a media presentationdescription (MPD). Manifest file 66 may contain descriptions ofdifferent alternative representations 68 (e.g., video services withdifferent qualities) and the description may include, e.g., codecinformation, a profile value, a level value, a bitrate, and otherdescriptive characteristics of representations 68. Client device 40 mayretrieve the MPD of a media presentation to determine how to accesssegments of representations 68.

In particular, retrieval unit 52 may retrieve configuration data (notshown) of client device 40 to determine decoding capabilities of videodecoder 48 and rendering capabilities of video output 44. Theconfiguration data may also include any or all of a language preferenceselected by a user of client device 40, one or more camera perspectivescorresponding to depth preferences set by the user of client device 40,and/or a rating preference selected by the user of client device 40.Retrieval unit 52 may comprise, for example, a web browser or a mediaclient configured to submit HTTP GET and partial GET requests. Retrievalunit 52 may correspond to software instructions executed by one or moreprocessors or processing units (not shown) of client device 40. In someexamples, all or portions of the functionality described with respect toretrieval unit 52 may be implemented in hardware, or a combination ofhardware, software, and/or firmware, where requisite hardware may beprovided to execute instructions for software or firmware.

Retrieval unit 52 may compare the decoding and rendering capabilities ofclient device 40 to characteristics of representations 68 indicated byinformation of manifest file 66. Retrieval unit 52 may initiallyretrieve at least a portion of manifest file 66 to determinecharacteristics of representations 68. For example, retrieval unit 52may request a portion of manifest file 66 that describes characteristicsof one or more adaptation sets. Retrieval unit 52 may select a subset ofrepresentations 68 (e.g., an adaptation set) having characteristics thatcan be satisfied by the coding and rendering capabilities of clientdevice 40. Retrieval unit 52 may then determine bitrates forrepresentations in the adaptation set, determine a currently availableamount of network bandwidth, and retrieve segments from one of therepresentations having a bitrate that can be satisfied by the networkbandwidth.

In general, higher bitrate representations may yield higher qualityvideo playback, while lower bitrate representations may providesufficient quality video playback when available network bandwidthdecreases. Accordingly, when available network bandwidth is relativelyhigh, retrieval unit 52 may retrieve data from relatively high bitraterepresentations, whereas when available network bandwidth is low,retrieval unit 52 may retrieve data from relatively low bitraterepresentations. In this manner, client device 40 may stream multimediadata over network 74 while also adapting to changing network bandwidthavailability of network 74.

Additionally or alternatively, retrieval unit 52 may be configured toreceive data in accordance with a broadcast or multicast networkprotocol, such as eMBMS or IP multicast. In such examples, retrievalunit 52 may submit a request to join a multicast network groupassociated with particular media content. After joining the multicastgroup, retrieval unit 52 may receive data of the multicast group withoutfurther requests issued to server device 60 or content preparationdevice 20. Retrieval unit 52 may submit a request to leave the multicastgroup when data of the multicast group is no longer needed, e.g., tostop playback or to change channels to a different multicast group.

As noted above, retrieval unit 52 of client device 40 may determine thatmanifest file 66 indicates that one or more segments of one or more ofrepresentations 68 are missing section segments. In response, retrievalunit 52 may avoid retrieval of the missing section segments as indicatedby manifest file 66. Additionally or alternatively, retrieval unit 52may determine that a retrieved segment is marked as a missing sectionsegment. Retrieval unit 52 may nevertheless extend a playback timelinewithout receiving an update to manifest file 66.

In some examples, content preparation device 20, server device 60, andclient device 40 may be configured according to modifications to DASH asdiscussed below:

5.3.9.7 Missing Section Signalling 5.3.9.7.1 General

In certain cases, a time continuous section of a Representation or anAdaptation Set is not properly represented, for example due to errors inthe content generation. However, at least on DASH level the contentshould be offered properly, for example by adding a Missing SectionSegment or by including content that is encoded but not representing theactual main media content.

The MissingSection element enables to express such sections that shouldbe avoided by the DASH client, for example by switching to aRepresentation that does not include such a Missing section.

The MissingSection element shall contain a list of S elements each ofwhich describes a sequence of contiguous Missing sections. The S elementcontains an optional @d attribute specifying the MPD duration of theMissing section and a @t time attribute whereby the value of the @tattribute minus the value of the @presentationTimeOffset specifies theMPD start time of the Missing section.

The semantics of the attributes and elements for Missing Section areprovided in 5.3.9.7.2, Table XX. The XML syntax of the Segment Timelineis provided in 5.3.9.7.3.

5.3.9.7.2 Semantics

TABLE XX Semantics of SegmentTimeline element Element or Attribute NameUse Description MissingSection specifies all Missing sections of theassociated Representation S 1 . . . N specifies one Missing section @t Mthis value of this attribute minus the value of the@presentationTimeOffset specifies the MPD start time, in @timescaleunits, of the first sample in the Missing section. The value of thisattribute must be equal to or greater than the sum of the previous Selement earliest presentation time and the sum of the contiguous sectiondurations. @d O specifies the Missing section duration in units of thevalue of the @timescale. If not present, the Missing section lasts untilthe end of the S element, the start of the next S element, the end ofthe Period or until the next MPD update. Legend: For attributes: M =Mandatory, O = Optional, OD = Optional with Default Value, CM =Conditionally Mandatory. For elements: <minOccurs> . . . <maxOccurs> (N= unbounded) Elements are bold; attributes are non-bold and precededwith an @.

5.3.9.7.3 XML Syntax

<!-- Segment Timeline --> <xs:complexType name=“MissingSectionType”> <xs:sequence>  <xs:element name=“S” minOccurs=“1”maxOccurs=“unbounded” >   <xs:complexType>   <xs:attribute name=“t”type=“xs:unsignedLong” use=“required”/>   <xs:attribute name=“d”type=“xs:unsignedLong”/>   <xs:anyAttribute namespace=“##other”processContents=“lax”/>   </xs:complexType>  </xs:element>  <xs:anynamespace=“##other” processContents=“lax” minOccurs=“0”maxOccurs=“unbounded”/>  </xs:sequence>  <xs:anyAttributenamespace=“##other” processContents=“lax”/> </xs:complexType>

Thus, content preparation device 20 and/or server device 60 may signal aMissingSection element of manifest file 66, which may be an MPD.Likewise, retrieval unit 52 of client device 40 may use theMissingSection element of manifest file 66 to determine one or moresegments (or portions of segments) of one or more of representations 68that are not to be retrieved. That is, the segments or portions thereofmay correspond to playback times for which no media data is available.

Additionally or alternatively, in some examples, content preparationdevice 20, server device 60, and client device 40 may be configuredaccording to modifications to DASH as discussed below.

The following is proposed specification text for MPEG-DASH ISO/IEC23009-1, according to which content preparation device 20, server device60, and client device 40 may be configured:

A Missing Section Segment is defined for the purpose to extend theSegment Timeline, even if the data in the Segment may not be present oris only partially present. The Segment may not contain any media data,i.e. neither a Movie Fragment Header nor a media data (mdat) box may bepresent. It may also contain only a subset of the data. If the Segmentis not a valid media segment and the content provider expects the DASHclient to signal the absence of media for the span, then the segmentshall include a major brand ‘miss’. The Segment may contain also an‘emsg’, for example, to indicate an MPD validity expiration. Inaddition, for a Missing Section segment, the following shall hold:

-   -   a single Segment Index (‘sidx’) box shall be present and the        values of the Segment Index shall describe accurate timing of        the Segment Timeline, i.e.:        -   the earliest_presentation_time in the ‘sidx’ box shall be            the value of S@t as signaled in the Segment Timeline in the            MPD.        -   the Subsegment_duration fields in the ‘sidx’ box shall be            the value of S@d as signaled in the Segment Timeline in the            MPD.

Note: by forcing these values, the DASH client is able to properlyextend the Segment Timeline even though no actual media data may bepresent.

The following is proposed specification text for DASH-IF IOP, accordingto which content preparation device 20, server device 60, and clientdevice 40 may be configured:

-   -   If the DASH client receives a segment with major brand ‘miss’,        the DASH client shall use the Segment Index (‘sidx’) box to        extend the timeline. If the segment does not contain all media        as indicated in the Segment Index, the DASH client should inform        the media decoder on missing information.

In this manner, content preparation device 20 and/or server device 60may signal data in a segment index (SIDX) box of a segment includingmissing sections, the data indicating an earliest_presentation_time andduration of the missing sections. Likewise, client device 40 maydetermine that the segment timeline is to be extended based on theinformation of the SIDX box. Additionally, in some examples, clientdevice 40 may retrieve alternative media data for the missing section ordetermine that no media data is to be played for the missing section.The alternative media data may be, for example, a black screen, one ormore error messages, or the like.

Network interface 54 may receive and provide data of segments of aselected representation to retrieval unit 52, which may in turn providethe segments to decapsulation unit 50. Decapsulation unit 50 maydecapsulate elements of a video file into constituent PES streams,depacketize the PES streams to retrieve encoded data, and send theencoded data to either audio decoder 46 or video decoder 48, dependingon whether the encoded data is part of an audio or video stream, e.g.,as indicated by PES packet headers of the stream. Audio decoder 46decodes encoded audio data and sends the decoded audio data to audiooutput 42, while video decoder 48 decodes encoded video data and sendsthe decoded video data, which may include a plurality of views of astream, to video output 44.

Video encoder 28, video decoder 48, audio encoder 26, audio decoder 46,encapsulation unit 30, retrieval unit 52, and decapsulation unit 50 eachmay be implemented as any of a variety of suitable processing circuitry,as applicable, such as one or more microprocessors, digital signalprocessors (DSPs), application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs), discrete logic circuitry,software, hardware, firmware or any combinations thereof. Each of videoencoder 28 and video decoder 48 may be included in one or more encodersor decoders, either of which may be integrated as part of a combinedvideo encoder/decoder (CODEC). Likewise, each of audio encoder 26 andaudio decoder 46 may be included in one or more encoders or decoders,either of which may be integrated as part of a combined CODEC. Anapparatus including video encoder 28, video decoder 48, audio encoder26, audio decoder 46, encapsulation unit 30, retrieval unit 52, and/ordecapsulation unit 50 may comprise an integrated circuit, amicroprocessor, and/or a wireless communication device, such as acellular telephone.

Client device 40, server device 60, and/or content preparation device 20may be configured to operate in accordance with the techniques of thisdisclosure. For purposes of example, this disclosure describes thesetechniques with respect to client device 40 and server device 60.However, it should be understood that content preparation device 20 maybe configured to perform these techniques, instead of (or in additionto) server device 60.

Encapsulation unit 30 may form NAL units comprising a header thatidentifies a program to which the NAL unit belongs, as well as apayload, e.g., audio data, video data, or data that describes thetransport or program stream to which the NAL unit corresponds. Forexample, in H.264/AVC, a NAL unit includes a 1-byte header and a payloadof varying size. A NAL unit including video data in its payload maycomprise various granularity levels of video data. For example, a NALunit may comprise a block of video data, a plurality of blocks, a sliceof video data, or an entire picture of video data. Encapsulation unit 30may receive encoded video data from video encoder 28 in the form of PESpackets of elementary streams. Encapsulation unit 30 may associate eachelementary stream with a corresponding program.

Encapsulation unit 30 may also assemble access units from a plurality ofNAL units. In general, an access unit may comprise one or more NAL unitsfor representing a frame of video data, as well as audio datacorresponding to the frame when such audio data is available. An accessunit generally includes all NAL units for one output time instance,e.g., all audio and video data for one time instance. For example, ifeach view has a frame rate of 20 frames per second (fps), then each timeinstance may correspond to a time interval of 0.05 seconds. During thistime interval, the specific frames for all views of the same access unit(the same time instance) may be rendered simultaneously. In one example,an access unit may comprise a coded picture in one time instance, whichmay be presented as a primary coded picture.

Accordingly, an access unit may comprise all audio and video frames of acommon temporal instance, e.g., all views corresponding to time X. Thisdisclosure also refers to an encoded picture of a particular view as a“view component.” That is, a view component may comprise an encodedpicture (or frame) for a particular view at a particular time.Accordingly, an access unit may be defined as comprising all viewcomponents of a common temporal instance. The decoding order of accessunits need not necessarily be the same as the output or display order.

A media presentation may include a media presentation description (MPD),which may contain descriptions of different alternative representations(e.g., video services with different qualities) and the description mayinclude, e.g., codec information, a profile value, and a level value. AnMPD is one example of a manifest file, such as manifest file 66. Clientdevice 40 may retrieve the MPD of a media presentation to determine howto access movie fragments of various presentations. Movie fragments maybe located in movie fragment boxes (moof boxes) of video files.

Manifest file 66 (which may comprise, for example, an MPD) may advertiseavailability of segments of representations 68. That is, the MPD mayinclude information indicating the wall-clock time at which a firstsegment of one of representations 68 becomes available, as well asinformation indicating the durations of segments within representations68. In this manner, retrieval unit 52 of client device 40 may determinewhen each segment is available, based on the starting time as well asthe durations of the segments preceding a particular segment.

After encapsulation unit 30 has assembled NAL units and/or access unitsinto a video file based on received data, encapsulation unit 30 passesthe video file to output interface 32 for output. In some examples,encapsulation unit 30 may store the video file locally or send the videofile to a remote server via output interface 32, rather than sending thevideo file directly to client device 40. Output interface 32 maycomprise, for example, a transmitter, a transceiver, a device forwriting data to a computer-readable medium such as, for example, anoptical drive, a magnetic media drive (e.g., floppy drive), a universalserial bus (USB) port, a network interface, or other output interface.Output interface 32 outputs the video file to a computer-readablemedium, such as, for example, a transmission signal, a magnetic medium,an optical medium, a memory, a flash drive, or other computer-readablemedium.

Network interface 54 may receive a NAL unit or access unit via network74 and provide the NAL unit or access unit to decapsulation unit 50, viaretrieval unit 52. Decapsulation unit 50 may decapsulate a elements of avideo file into constituent PES streams, depacketize the PES streams toretrieve encoded data, and send the encoded data to either audio decoder46 or video decoder 48, depending on whether the encoded data is part ofan audio or video stream, e.g., as indicated by PES packet headers ofthe stream. Audio decoder 46 decodes encoded audio data and sends thedecoded audio data to audio output 42, while video decoder 48 decodesencoded video data and sends the decoded video data, which may include aplurality of views of a stream, to video output 44.

FIG. 2 is a block diagram illustrating an example set of components ofretrieval unit 52 of FIG. 1 in greater detail. In this example,retrieval unit 52 includes eMBMS middleware unit 100, DASH client 110,and media application 112.

In this example, eMBMS middleware unit 100 further includes eMBMSreception unit 106, cache 104, and proxy server unit 102. In thisexample, eMBMS reception unit 106 is configured to receive data viaeMBMS, e.g., according to File Delivery over Unidirectional Transport(FLUTE), described in T. Paila et al., “FLUTE—File Delivery overUnidirectional Transport,” Network Working Group, RFC 6726, November2012, available at http://tools.ietf.org/html/rfc6726. That is, eMBMSreception unit 106 may receive files via broadcast from, e.g., serverdevice 60, which may act as a broadcast/multicast service center(BM-SC).

As eMBMS middleware unit 100 receives data for files, eMBMS middlewareunit may store the received data in cache 104. Cache 104 may comprise acomputer-readable storage medium, such as flash memory, a hard disk,RAM, or any other suitable storage medium.

Proxy server unit 102 may act as a server for DASH client 110. Forexample, proxy server unit 102 may provide an MPD file or other manifestfile to DASH client 110. Proxy server unit 102 may advertiseavailability times for segments in the MPD file, as well as hyperlinksfrom which the segments can be retrieved. These hyperlinks may include alocalhost address prefix corresponding to client device 40 (e.g.,127.0.0.1 for IPv4). In this manner, DASH client 110 may requestsegments from proxy server unit 102 using HTTP GET or partial GETrequests. For example, for a segment available from linkhttp://127.0.0.1/rep1/seg3, DASH client 110 may construct an HTTP GETrequest that includes a request for http://127.0.0.1/rep1/seg3, andsubmit the request to proxy server unit 102. Proxy server unit 102 mayretrieve requested data from cache 104 and provide the data to DASHclient 110 in response to such requests.

FIG. 3 is a conceptual diagram illustrating elements of examplemultimedia content 120. Multimedia content 120 may correspond tomultimedia content 64 (FIG. 1), or another multimedia content stored instorage medium 62. In the example of FIG. 3, multimedia content 120includes media presentation description (MPD) 122 and a plurality ofrepresentations 124A-124N (representations 124). Representation 124Aincludes optional header data 126 and segments 128A-128N (segments 128),while representation 124N includes optional header data 130 and segments132A-132N (segments 132). The letter N is used to designate the lastmovie fragment in each of representations 124 as a matter ofconvenience. In some examples, there may be different numbers of moviefragments between representations 124.

MPD 122 may comprise a data structure separate from representations 124.MPD 122 may correspond to manifest file 66 of FIG. 1. Likewise,representations 124 may correspond to representations 68 of FIG. 1. Ingeneral, MPD 122 may include data that generally describescharacteristics of representations 124, such as coding and renderingcharacteristics, adaptation sets, a profile to which MPD 122corresponds, text type information, camera angle information, ratinginformation, trick mode information (e.g., information indicative ofrepresentations that include temporal sub-sequences), and/or informationfor retrieving remote periods (e.g., for targeted advertisementinsertion into media content during playback).

Header data 126, when present, may describe characteristics of segments128, e.g., temporal locations of random access points (RAPs, alsoreferred to as stream access points (SAPs)), which of segments 128includes random access points, byte offsets to random access pointswithin segments 128, uniform resource locators (URLs) of segments 128,or other aspects of segments 128. Header data 130, when present, maydescribe similar characteristics for segments 132. Additionally oralternatively, such characteristics may be fully included within MPD122.

Segments 128, 132 include one or more coded video samples, each of whichmay include frames or slices of video data. Each of the coded videosamples of segments 128 may have similar characteristics, e.g., height,width, and bandwidth requirements. Such characteristics may be describedby data of MPD 122, though such data is not illustrated in the exampleof FIG. 3. MPD 122 may include characteristics as described by the 3GPPSpecification, with the addition of any or all of the signaledinformation described in this disclosure.

Each of segments 128, 132 may be associated with a unique uniformresource locator (URL). Thus, each of segments 128, 132 may beindependently retrievable using a streaming network protocol, such asDASH. In this manner, a destination device, such as client device 40,may use an HTTP GET request to retrieve segments 128 or 132. In someexamples, client device 40 may use HTTP partial GET requests to retrievespecific byte ranges of segments 128 or 132.

In accordance with the techniques of this disclosure, MPD 122 mayinclude a MissingSection element as discussed above, to indicate thatone or more of segments 128 and/or 132 are missing section segments. Ifsegment 128B is a missing section segment, for example, client device 40may alternatively retrieve segment 132B, which has a playback timeoverlapping the missing section of representation 124A (i.e., segment128B). In this example, segments 128B and 132B include media data havingthe same playback time, also referred to herein as presentation time.Thus, media data of segment 132B can act as replacement media data forsegment 128B when segment 128B includes a missing section (e.g., some orall of the media data of segment 128B is missing and cannot beretrieved). Client device 40 may select segment 132B as a replacementsegment for segment 128B based on representation 124N and representation124A being in the same adaptation set, for example.

FIG. 4 is a block diagram illustrating elements of an example video file150, which may correspond to a segment of a representation, such as oneof segments 128, 132 of FIG. 3. Each of segments 128, 132 may includedata that conforms substantially to the arrangement of data illustratedin the example of FIG. 4. Video file 150 may be said to encapsulate asegment. As described above, video files in accordance with the ISO basemedia file format and extensions thereof store data in a series ofobjects, referred to as “boxes.” In the example of FIG. 4, video file150 includes file type (FTYP) box 152, movie (MOOV) box 154, segmentindex (SIDX) boxes 162, movie fragment (MOOF) boxes 164, and moviefragment random access (MFRA) box 166. Although FIG. 4 represents anexample of a video file, it should be understood that other media filesmay include other types of media data (e.g., audio data, timed textdata, or the like) that is structured similarly to the data of videofile 150, in accordance with the ISO base media file format and itsextensions.

File type (FTYP) box 152 generally describes a file type for video file150. File type box 152 may include data that identifies a specificationthat describes a best use for video file 150. File type box 152 mayalternatively be placed before MOOV box 154, movie fragment boxes 164,and/or MFRA box 166.

In some examples, a segment, such as video file 150, may include an MPDupdate box (not shown) before FTYP box 152. The MPD update box mayinclude information indicating that an MPD corresponding to arepresentation including video file 150 is to be updated, along withinformation for updating the MPD. For example, the MPD update box mayprovide a URI or URL for a resource to be used to update the MPD. Asanother example, the MPD update box may include data for updating theMPD. In some examples, the MPD update box may immediately follow asegment type (STYP) box (not shown) of video file 150, where the STYPbox may define a segment type for video file 150.

MOOV box 154, in the example of FIG. 4, includes movie header (MVHD) box156, missing section (MS) box 157, track (TRAK) box 158, and one or moremovie extends (MVEX) boxes 160. In general, MVHD box 156 may describegeneral characteristics of video file 150. For example, MVHD box 156 mayinclude data that describes when video file 150 was originally created,when video file 150 was last modified, a timescale for video file 150, aduration of playback for video file 150, or other data that generallydescribes video file 150.

MS box 157 may include information indicating that one or more sectionsof video file 150 are not properly represented, e.g., due to errors incontent generation. MS box 157 allows a client device, such as clientdevice 40, to avoid such sections, e.g., by switching to arepresentation that does not include such a missing section. MS box 157may include a list of S elements, each of which may describe a sequenceof contiguous missing sections. The S element may contain an optional @dattribute specifying the MPD duration of the missing section and an @ttime attribute having a value from which the value of@presentationTimeOffset can be subtracted to specify the MPD start timeof the missing section. MS box 157 may be included in a SegmentTimelineelement (not shown) of video file 150, which may be included in MOOV box154.

TRAK box 158 may include data for a track of video file 150. TRAK box158 may include a track header (TKHD) box that describes characteristicsof the track corresponding to TRAK box 158. In some examples, TRAK box158 may include coded video pictures, while in other examples, the codedvideo pictures of the track may be included in movie fragments 164,which may be referenced by data of TRAK box 158 and/or SIDX boxes 162.

In some examples, video file 150 may include more than one track.Accordingly, MOOV box 154 may include a number of TRAK boxes equal tothe number of tracks in video file 150. TRAK box 158 may describecharacteristics of a corresponding track of video file 150. For example,TRAK box 158 may describe temporal and/or spatial information for thecorresponding track. A TRAK box similar to TRAK box 158 of MOOV box 154may describe characteristics of a parameter set track, whenencapsulation unit 30 (FIG. 3) includes a parameter set track in a videofile, such as video file 150. Encapsulation unit 30 may signal thepresence of sequence level SEI messages in the parameter set trackwithin the TRAK box describing the parameter set track.

MVEX boxes 160 may describe characteristics of corresponding moviefragments 164, e.g., to signal that video file 150 includes moviefragments 164, in addition to video data included within MOOV box 154,if any. In the context of streaming video data, coded video pictures maybe included in movie fragments 164 rather than in MOOV box 154.Accordingly, all coded video samples may be included in movie fragments164, rather than in MOOV box 154.

MOOV box 154 may include a number of MVEX boxes 160 equal to the numberof movie fragments 164 in video file 150. Each of MVEX boxes 160 maydescribe characteristics of a corresponding one of movie fragments 164.For example, each MVEX box may include a movie extends header box (MEHD)box that describes a temporal duration for the corresponding one ofmovie fragments 164.

As noted above, encapsulation unit 30 may store a sequence data set in avideo sample that does not include actual coded video data. A videosample may generally correspond to an access unit, which is arepresentation of a coded picture at a specific time instance. In thecontext of AVC, the coded picture include one or more VCL NAL units,which contain the information to construct all the pixels of the accessunit and other associated non-VCL NAL units, such as SEI messages.Accordingly, encapsulation unit 30 may include a sequence data set,which may include sequence level SEI messages, in one of movie fragments164. Encapsulation unit 30 may further signal the presence of a sequencedata set and/or sequence level SEI messages as being present in one ofmovie fragments 164 within the one of MVEX boxes 160 corresponding tothe one of movie fragments 164.

SIDX boxes 162 are optional elements of video file 150. That is, videofiles conforming to the 3GPP file format, or other such file formats, donot necessarily include SIDX boxes 162. In accordance with the exampleof the 3GPP file format, a SIDX box may be used to identify asub-segment of a segment (e.g., a segment contained within video file150). The 3GPP file format defines a sub-segment as “a self-containedset of one or more consecutive movie fragment boxes with correspondingMedia Data box(es) and a Media Data Box containing data referenced by aMovie Fragment Box must follow that Movie Fragment box and precede thenext Movie Fragment box containing information about the same track.”The 3GPP file format also indicates that a SIDX box “contains a sequenceof references to subsegments of the (sub)segment documented by the box.The referenced subsegments are contiguous in presentation time.Similarly, the bytes referred to by a Segment Index box are alwayscontiguous within the segment. The referenced size gives the count ofthe number of bytes in the material referenced.”

SIDX boxes 162 generally provide information representative of one ormore sub-segments of a segment included in video file 150. For instance,such information may include playback times at which sub-segments beginand/or end, byte offsets for the sub-segments, whether the sub-segmentsinclude (e.g., start with) a stream access point (SAP), a type for theSAP (e.g., whether the SAP is an instantaneous decoder refresh (IDR)picture, a clean random access (CRA) picture, a broken link access (BLA)picture, or the like), a position of the SAP (in terms of playback timeand/or byte offset) in the sub-segment, and the like.

In accordance with the techniques of this disclosure, if video file 150includes a missing section, video file 150 may include a single SIDX box162 having an earliest_presentation_time element having the value of S@tas signaled in the manifest file (e.g., MPD) and a Subsegment_durationfield having the value of S@d as signaled in the manifest file (e.g.,MPD). In some examples, video file 150 may include only SIDX box 162,without MOOV box 154 and/or movie fragments 164. In other examples,video file 150 may include both media data (e.g., movie fragments 164)and one or more SIDX boxes 162 signaling a missing section for which nomovie fragments 164 are available.

Movie fragments 164 may include one or more coded video pictures. Insome examples, movie fragments 164 may include one or more groups ofpictures (GOPs), each of which may include a number of coded videopictures, e.g., frames or pictures. In addition, as described above,movie fragments 164 may include sequence data sets in some examples.Each of movie fragments 164 may include a movie fragment header box(MFHD, not shown in FIG. 4). The MFHD box may describe characteristicsof the corresponding movie fragment, such as a sequence number for themovie fragment. Movie fragments 164 may be included in order of sequencenumber in video file 150. Movie fragments 164 may not be present invideo file 150 when video file 150 is a missing section segment.

MFRA box 166 may describe random access points within movie fragments164 of video file 150. This may assist with performing trick modes, suchas performing seeks to particular temporal locations (i.e., playbacktimes) within a segment encapsulated by video file 150. MFRA box 166 isgenerally optional and need not be included in video files, in someexamples. Likewise, a client device, such as client device 40, does notnecessarily need to reference MFRA box 166 to correctly decode anddisplay video data of video file 150. MFRA box 166 may include a numberof track fragment random access (TFRA) boxes (not shown) equal to thenumber of tracks of video file 150, or in some examples, equal to thenumber of media tracks (e.g., non-hint tracks) of video file 150.

In some examples, movie fragments 164 may include one or more streamaccess points (SAPs), such as IDR pictures. Likewise, MFRA box 166 mayprovide indications of locations within video file 150 of the SAPs.Accordingly, a temporal sub-sequence of video file 150 may be formedfrom SAPs of video file 150. The temporal sub-sequence may also includeother pictures, such as P-frames and/or B-frames that depend from SAPs.Frames and/or slices of the temporal sub-sequence may be arranged withinthe segments such that frames/slices of the temporal sub-sequence thatdepend on other frames/slices of the sub-sequence can be properlydecoded. For example, in the hierarchical arrangement of data, data usedfor prediction for other data may also be included in the temporalsub-sequence. MFRA box 166 may not be present in video file 150 whenvideo file 150 is a missing section segment.

FIG. 5 is a conceptual diagram illustrating various examples ofscenarios in which loss of media data of one or more segments may occur.In various examples, part of a segment may be lost (and thus not provideproper content), a full segment may be lost, or multiple segments may belost. In some cases, it may also be the case that only the initial partof a Segment can be produced. Some examples are shown in FIG. 5.

In particular, FIG. 5 depicts a first example sequence of segments 200including segments 202A-202H, in which there is no actual or concealedloss. FIG. 5 also depicts a second example sequence of segments 204including segments 206A-206H, in which segment 206C includes a missingsection 208 due to loss of media data. FIG. 5 also depicts a thirdexample sequence of segments 210 including segments 212A-212H, in whichthere is loss at the end of segment 212E, such that segment 212Eincludes missing section 214. FIG. 5 also depicts a fourth examplesequence of segments 216 including segments 218A-218H, in which there isloss at an anticipated segment boundary of segment 218D that includesmissing section 220, which may cause segment 218D not to start at apredicted time. FIG. 5 also depicts a fifth example sequence of segments222 including segments 224A-2224C and 224E-224H in which there is a lossresulting in missing section 226 at the segment boundary for a missingsegment 228 following segment 224C and before segment 224E. FIG. 5 alsodepicts a sixth example sequence of segments 230 including segments232A-232C, 232G, and 232H, in which there is a loss of media dataresulting in missing section 234 across multiple segments.

In accordance with the techniques of this disclosure, a manifest filemay include a Missing Section element to signal any of missing sections208, 214, 220, 226, and/or 234. Likewise, segment 206C may include data(such as a SIDX box) to signal missing section 208; segment 212E mayinclude data to signal missing section 214; and segment 218D may includedata to signal missing section 220.

FIG. 6 is a flowchart illustrating an example method of determining thatstreamed media data includes a missing section according to thetechniques of this disclosure. The method of FIG. 6 may be performed by,e.g., retrieval unit 52 of client device 40 of FIG. 1, for example.However, it should be understood that other devices may be configured toperform this or a similar method in accordance with the techniques ofthis disclosure.

Initially, retrieval unit 52 may retrieve a manifest file for apresentation of media data (250). The manifest file may be, for example,a media presentation of DASH. In other examples, other manifest filesfor other streaming protocols, such as HTTP Live Streaming (HLS), AdobeHTTP Dynamic Streaming (HDS), Microsoft Smooth Streaming (MSS), or thelike.

The manifest file may include data indicating network locations (e.g.,URLs) of segments of media data. Thus, retrieval unit 52 may use themanifest file to retrieve one or more of the segments of thepresentation (252). In particular, as explained above, retrieval unit 52may determine coding and rendering capabilities of client device 40, aswell as a currently available network bandwidth, to select an adaptationset and a representation within the adaptation set, then determine theURLs of segments of the selected representation, and send requests forthe segments.

In some examples, the manifest file may include a Missing Sectionelement, as discussed above. In particular, the manifest file mayinclude one or more Missing Section elements, each of which may signal astart time and duration of a corresponding period of playback time forwhich media data is not available. Additionally or alternatively, one ormore of the segments may include data indicating that the segmentincludes a missing section. For example, the segment may include asegment index (SIDX) box indicating a starting time and a duration ofthe missing section of the segment. Accordingly, using the manifest fileand/or the data of one or more of the segments themselves, retrievalunit 52 may determine that the segment includes a missing section (254).

In response to determining that the segment includes the missingsection, and using the data indicative of the missing section (e.g., astart time and duration of the missing section), retrieval unit 52 mayextend the segment timeline (256) to account for the missing section. Inparticular, retrieval unit 52 may determine that no media data isavailable for the missing section, and thus, retrieval unit 52 may avoidattempting to retrieve additional media data for the missing section. Insome examples, retrieval unit 52 may retrieve alternative, replacementmedia data for the missing section, such as a black screen or an errormessage.

Retrieval unit 52 may further proceed to retrieve subsequent segmentsfollowing the missing section (258). Retrieval unit 52 may also outputmedia data of the retrieved segments (260). In particular, retrievalunit 52 may output media data of the retrieved segments that is outsideof the missing section. In some cases, retrieval unit 52 may output thereplacement media data for the missing section.

In this manner, the method of FIG. 6 represents an example of a methodincluding determining that a manifest file for a presentation of mediadata includes data specifying a period of time of the presentation forwhich media data is not available but for which a segment timeline canbe extended; retrieving a first set of segments of the presentationincluding media data having presentation times prior to the period oftime; in response to the determination that the manifest file includesthe data specifying the period of time: omitting retrieval of media datahaving presentation times during the period of time; retrieving a secondset of segments of the presentation including media data havingpresentation times after the period of time; and extending the segmenttimeline according to the determination.

Additionally, the method of FIG. 6 also represents an example of amethod including retrieving at least a portion of a first segment of arepresentation of media data, the at least portion of the first segmentincluding data indicating that the first segment includes a missingsection representing a period of time for which media data is notavailable but for which a segment timeline can be extended; retrievingmedia data of a second segment having a presentation time following themissing section; extending the segment timeline according to the data ofthe at least portion of the first segment; and outputting the media dataof the second segment according to the extended segment timeline.

FIG. 7 is a flowchart illustrating an example of a method for retrievinga manifest file including data indicating that a presentation includes amissing section, according to the techniques of this disclosure. Themethod of FIG. 7 may be performed by, e.g., retrieval unit 52 of clientdevice 40 of FIG. 1, for example. However, it should be understood thatother devices may be configured to perform this or a similar method inaccordance with the techniques of this disclosure.

Initially, retrieval unit 52 may retrieve a manifest file for apresentation of media data (270). The manifest file may be, for example,a media presentation of DASH. In other examples, other manifest filesfor other streaming protocols, such as HTTP Live Streaming (HLS), AdobeHTTP Dynamic Streaming (HDS), Microsoft Smooth Streaming (MSS), or thelike.

The manifest file may include data indicating network locations (e.g.,URLs) of segments of media data. Thus, retrieval unit 52 may use themanifest file to retrieve one or more of the segments of thepresentation. In particular, as explained above, retrieval unit 52 maydetermine coding and rendering capabilities of client device 40, as wellas a currently available network bandwidth, to select an adaptation setand a representation within the adaptation set, then determine the URLsof segments of the selected representation, and send requests for thesegments.

Additionally, in this example, retrieval unit 52 determines that themanifest file includes a Missing Section element (272). For example, theMissing Section element may signal a start time and a duration of aperiod of time for which media data is not available. Thus, retrievalunit 52 may determine the start time of the missing section (274) and aduration of the missing section (276) using the manifest file. The starttime may be indicated by an S@t element of the manifest file, while theduration may be indicated by an S@d element of the manifest file.

Retrieval unit 52 may then extend the segment timeline according to theMissing Section element of the manifest file (278). Although not shownin FIG. 7 explicitly,

In this manner, the method of FIG. 7 represents an example of a methodincluding determining that a manifest file for a presentation of mediadata includes data specifying a period of time of the presentation forwhich media data is not available but for which a segment timeline canbe extended; retrieving a first set of segments of the presentationincluding media data having presentation times prior to the period oftime; in response to the determination that the manifest file includesthe data specifying the period of time: omitting retrieval of media datahaving presentation times during the period of time; retrieving a secondset of segments of the presentation including media data havingpresentation times after the period of time; and extending the segmenttimeline according to the determination.

FIG. 8 is a flowchart illustrating an example of a method for retrievinga segment including data indicating that the segment includes a missingsection, according to the techniques of this disclosure. The method ofFIG. 8 may be performed by, e.g., retrieval unit 52 of client device 40of FIG. 1, for example. However, it should be understood that otherdevices may be configured to perform this or a similar method inaccordance with the techniques of this disclosure.

Initially, retrieval unit 52 may retrieve a manifest file for apresentation of media data (290). The manifest file may be, for example,a media presentation of DASH. In other examples, other manifest filesfor other streaming protocols, such as HTTP Live Streaming (HLS), AdobeHTTP Dynamic Streaming (HDS), Microsoft Smooth Streaming (MSS), or thelike.

The manifest file may include data indicating network locations (e.g.,URLs) of segments of media data. Thus, retrieval unit 52 may use themanifest file to retrieve segments of the presentation (292). Inparticular, as explained above, retrieval unit 52 may determine codingand rendering capabilities of client device 40, as well as a currentlyavailable network bandwidth, to select an adaptation set and arepresentation within the adaptation set, then determine the URLs ofsegments of the selected representation, and send HTTP Get or partialGet requests for the segments.

Retrieval unit 52 may then determine that a nominal first retrievedsegment includes an indication of a missing section (294). That is, thefirst segment may be retrieved after other segments, but is referred toas “first” nominally to refer specifically to a segment having dataindicating that the segment includes a missing section. The firstsegment may include a SIDX box indicating a start time and duration ofthe missing section. Retrieval unit 52 may determine a subsegment forwhich no media is available using, e.g., an earliest_presentation timefield and a subsegment_duration field of the SIDX box of the firstsegment. The first segment may further include a major brand of “miss,”to indicate that the first segment includes the SIDX box signaling themissing section thereof. Moreover, retrieval unit 52 may extract anevent message (emsg) element from the segment, indicating expiration ofthe manifest file. Thus, retrieval unit 52 may use the data of the emsgelement to retrieve a new, valid manifest file.

Retrieval unit 52 may use the indication of the missing section toextend the segment timeline (296), that is, to avoid attempting toretrieve media data for the missing section and without sending actualmedia data of the first segment for the missing section to, e.g., avideo decoder (such as video decoder 48 of FIG. 1). In some examples,retrieval unit 52 may output replacement media data, such as a blackscreen or error message, and additionally or alternatively output anindication of the missing section to, e.g., video decoder 48. To obtainthe replacement media data, retrieval unit 52 may retrieve a segment ofa different representation corresponding to the missing section.

Alternatively, retrieval unit 52 may determine that a replacementsegment of a different representation includes valid media data havingthe same presentation time as the missing of the first segment. Thus,retrieval unit 52 may retrieve the replacement segment in response todetermining that the first segment includes the missing section. Thedifferent representation may be within the same adaptation set, buthave, e.g., a lower bitrate than the representation including the firstsegment. In cases where retrieval unit 52 retrieves the replacementsegment from a different representation, retrieval unit 52 may alsooutput the media data of the replacement segment to extend the segmenttimeline.

Furthermore, retrieval unit 52 may output media data of the segmentsthat is outside of the missing section according to the extended segmenttimeline (298). For example, retrieval unit 52 may output media datahaving presentation times preceding the missing section and media datahaving presentation times following the missing section to video decoder48, where the media data may be included in the first segment or asecond, subsequent segment (in presentation time).

In this manner, the method of FIG. 8 represents an example of a methodincluding retrieving at least a portion of a first segment of arepresentation of media data, the at least portion of the first segmentincluding data indicating that the first segment includes a missingsection representing a period of time for which media data is notavailable but for which a segment timeline can be extended; retrievingmedia data of a second segment having a presentation time following themissing section; extending the segment timeline according to the data ofthe at least portion of the first segment; and outputting the media dataof the second segment according to the extended segment timeline.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, code,and/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of retrieving media data, the methodcomprising: determining that a manifest file for a presentation of mediadata includes data specifying a period of time of the presentation forwhich media data is not available but for which a segment timeline canbe extended; retrieving a first set of segments of the presentationincluding media data having presentation times prior to the period oftime; retrieving a second set of segments of the presentation includingmedia data having presentation times after the period of time; inresponse to the determination that the manifest file includes the dataspecifying the period of time: omitting retrieval of media data havingpresentation times during the period of time; and extending the segmenttimeline according to the determination.
 2. The method of claim 1,wherein the manifest file comprises a media presentation description(MPD).
 3. The method of claim 1, wherein the data specifying the periodof time for which the media data is not available comprises aMissingSection element of the manifest file.
 4. The method of claim 1,wherein the data specifying the period of time for which the media datais not available comprises an attribute having a value representing astart time of the period of time for which the media data is notavailable.
 5. The method of claim 4, further comprising determining thestart time of the period of time for which the media data is notavailable using the value of the attribute, comprising subtracting avalue of a presentation time offset element from the value of theattribute to determine the start time of the period of time for whichthe media data is not available.
 6. The method of claim 4, wherein theattribute comprises an @t attribute.
 7. The method of claim 1, whereinthe data specifying the period of time for which the media data is notavailable comprises an attribute having a value specifying a duration ofthe period of time for which the media data is not available.
 8. Themethod of claim 7, wherein the attribute comprises an @d attribute.
 9. Adevice for retrieving media data, the device comprising: a memoryconfigured to store media data; and one or more processors implementedin circuitry and configured to: determine that a manifest file for apresentation of the media data includes data specifying a period of timeof the presentation for which media data is not available but for whicha segment timeline can be extended; retrieve a first set of segments ofthe presentation including media data having presentation times prior tothe period of time; retrieve a second set of segments of thepresentation including media data having presentation times after theperiod of time; in response to the determination that the manifest fileincludes the data specifying the period of time: omit retrieval of mediadata having presentation times during the period of time; and extend thesegment timeline according to the determination.
 10. The device of claim9, wherein the manifest file comprises a media presentation description(MPD).
 11. The device of claim 9, wherein the data specifying the periodof time for which the media data is not available comprises aMissingSection element of the manifest file.
 12. The device of claim 9,wherein the data specifying the period of time for which the media datais not available comprises an attribute having a value representing astart time of the period of time for which the media data is notavailable.
 13. The device of claim 12, wherein the one or moreprocessors are further configured to determine the start time of theperiod of time for which the media data is not available using the valueof the attribute, comprising subtracting a value of a presentation timeoffset element from the value of the attribute to determine the starttime of the period of time for which the media data is not available.14. The device of claim 12, wherein the attribute comprises an @tattribute.
 15. The device of claim 9, wherein the data specifying theperiod of time for which the media data is not available comprises anattribute having a value specifying a duration of the period of time forwhich the media data is not available.
 16. The device of claim 15,wherein the attribute comprises an @d attribute.
 17. A computer-readablestorage medium having stored thereon instructions that, when executed,cause a processor to: determine that a manifest file for a presentationof media data includes data specifying a period of time of thepresentation for which media data is not available but for which asegment timeline can be extended; retrieve a first set of segments ofthe presentation including media data having presentation times prior tothe period of time; retrieve a second set of segments of thepresentation including media data having presentation times after theperiod of time; in response to the determination that the manifest fileincludes the data specifying the period of time: omit retrieval of mediadata having presentation times during the period of time; and extend thesegment timeline according to the determination.
 18. Thecomputer-readable storage medium of claim 17, wherein the manifest filecomprises a media presentation description (MPD).
 19. Thecomputer-readable storage medium of claim 17, wherein the dataspecifying the period of time for which the media data is not availablecomprises a MissingSection element of the manifest file.
 20. Thecomputer-readable storage medium of claim 17, wherein the dataspecifying the period of time for which the media data is not availablecomprises an attribute having a value representing a start time of theperiod of time for which the media data is not available.
 21. Thecomputer-readable storage medium of claim 20, further comprisinginstructions that cause the processor to determine the start time of theperiod of time for which the media data is not available using the valueof the attribute, comprising subtracting a value of a presentation timeoffset element from the value of the attribute to determine the starttime of the period of time for which the media data is not available.22. The computer-readable storage medium of claim 20, wherein theattribute comprises an @t attribute.
 23. The computer-readable storagemedium of claim 17, wherein the data specifying the period of time forwhich the media data is not available comprises an attribute having avalue specifying a duration of the period of time for which the mediadata is not available.
 24. The computer-readable storage medium of claim23, wherein the attribute comprises an @d attribute.
 25. A device forretrieving media data, the device comprising: means for determining thata manifest file for a presentation of media data includes dataspecifying a period of time of the presentation for which media data isnot available but for which a segment timeline can be extended; meansfor retrieving a first set of segments of the presentation includingmedia data having presentation times prior to the period of time; meansfor retrieving a second set of segments of the presentation includingmedia data having presentation times after the period of time; means foromitting retrieval of media data having presentation times during theperiod of time in response to the determination that the manifest fileincludes the data specifying the period of time; and means for extendingthe segment timeline according to the determination in response to thedetermination that the manifest file includes the data specifying theperiod of time.
 26. The device of claim 25, wherein the manifest filecomprises a media presentation description (MPD).
 27. The device ofclaim 25, wherein the data specifying the period of time for which themedia data is not available comprises a MissingSection element of themanifest file.
 28. The device of claim 25, wherein the data specifyingthe period of time for which the media data is not available comprisesan attribute having a value representing a start time of the period oftime for which the media data is not available.
 29. The device of claim28, further comprising means for determining the start time of theperiod of time for which the media data is not available using the valueof the attribute, comprising subtracting a value of a presentation timeoffset element from the value of the attribute to determine the starttime of the period of time for which the media data is not available.30. The device of claim 28, wherein the attribute comprises an @tattribute.
 31. The device of claim 25, wherein the data specifying theperiod of time for which the media data is not available comprises anattribute having a value specifying a duration of the period of time forwhich the media data is not available.
 32. The device of claim 31,wherein the attribute comprises an @d attribute.